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Contraction- and hypoxia-stimulated glucose transport is mediated by a Ca²⁺-dependent mechanism in slow-twitch rat soleus muscle

Division of Geriatrics and Nutritional Sciences, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri

Submitted 29 November 2004 ; accepted in final form 11 January 2005

Increases in contraction-stimulated glucose transport in fast-twitch rat epitrochlearis muscle are mediated by AMPK- and Ca²⁺/calmodulin-dependent protein kinase (CAMK)-dependent signaling pathways. However, recent studies provide evidence suggesting that contraction-stimulated glucose transport in slow-twitch skeletal muscle is mediated through an AMPK-independent pathway. The purpose of the present study was to test the hypothesis that contraction-stimulated glucose transport in rat slow-twitch soleus muscle is mediated by an AMPK-independent/Ca²⁺-dependent pathway. Caffeine, a sarcoplasmic reticulum (SR) Ca²⁺-releasing agent, at a concentration that does not cause muscle contractions or decreases in high-energy phosphates, led to an 2-fold increase in 2-deoxyglucose (2-DG) uptake in isolated split soleus muscles. This increase in glucose transport was prevented by the SR calcium channel blocker dantrolene and the CAMK inhibitor KN93. Conversely, 5-aminoimidazole-4-carboxamide-1--D-ribofuranoside (AICAR), an AMPK activator, had no effect on 2-DG uptake in isolated split soleus muscles yet resulted in an 2-fold increase in the phosphorylation of AMPK and its downstream substrate acetyl-CoA carboxylase. The hypoxia-induced increase in 2-DG uptake was prevented by dantrolene and KN93, whereas hypoxia-stimulated phosphorylation of AMPK was unaltered by these agents. Tetanic muscle contractions resulted in an 3.5-fold increase in 2-DG uptake that was prevented by KN93, which did not prevent AMPK phosphorylation. Taken in concert, our results provide evidence that hypoxia- and contraction-stimulated glucose transport is mediated entirely through a Ca²⁺-dependent mechanism in rat slow-twitch muscle.

in vitro; rodent; metabolism; exercise

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